1. Field of the Invention
The present invention relates to a data transmission system and, in particular, it relates to a transmission apparatus and a method for transmitting data, in a data transmission system. The transmission apparatuses are interconnected for transmitting synchronous data through transmission lines each having a redundant structure. Further, at least one terminal is connected to each transmission apparatus for transmitting or receiving asynchronous data.
2. Description of the Related Art
There is a known data transmission system having the following structure. That is, at least one terminal is connected to a transmission apparatus through an asynchronous network, for example, a local area network (LAN). Further, a plurality of transmission apparatuses are interconnected through transmission lines each formed by a synchronous network. The synchronous network has a redundant structure duplexed by a current line (0-line) and a spare line (1-line). In this case, the data is transmitted or received between the terminal and the transmission apparatus based on the asynchronous data format, and is transmitted or received between transmission apparatuses based on the synchronous data format.
Further, the transmission apparatus at a sending side transmits the same synchronous data on both the current line and spare line. The transmission apparatus at a receiving side receives the synchronous data through the current line. If a failure occurs on the current line, the transmission apparatus at the receiving side switches the transmission line to the spare line in order to receive the synchronous data.
There are, however, some problems in the above-conventional art as explained in detail with reference to the attached drawings. Briefly, as a first problem, a long switching time is needed to switch a line from an abnormal line to a normal line when transmitting the asynchronous data. As a second problem, normal data transmission is interrupted for a long time until another Spanning Tree is re-constructed. As a third problem, the cost of transmission apparatus becomes high because it is necessary to provide a high performance apparatus in order to execute a learning function based on the destination MAC address. Finally, as a fourth problem, safety of data transmission cannot be ensured because a newly learned address is informed from one transmission apparatus to another apparatuses by using broadcast, and there may be some apparatuses which cannot receive the newly learned address.